"Dave S" wrote in message
. net...
Gary... I was looking for actual formalas.. not wild ass guesses or
rough approximations.. Computer spreadsheets use mathematical equations.
This is something that may be used by others besides myself. I
understand the concepts.. I DIDNT have the actual calcs on hand when I
posted my request.

Sorry, I didn't mean to be unresponsive--saying that the speed's
proportionate to the square root of gross weight _is_ the actual formula
(just expressed in English, and trivially translatable into mathematical
symbols or spreadsheet expressions--it's like saying "take the sum of the
passengers' weights" instead of saying "Pax1weight+Pax2weight").

As for approximations, the point of the alternate formula I gave is that it
gives a very _close_ approximation, not a rough approximation or wild-ass
guess. Unless you can control your airspeed to a fraction of a knot, the
approximation is just as good as the exact answer. Knowing the
approximation is important so that 1) you can quickly and easily
sanity-check what your spreadsheet tells you; and 2) if you realize someday
that you neglected to pre-calculate your V-speeds, or that you calculated
them based on weight assumptions that later changed, you can then
re-calculate in your head while you're flying (it's a lot easier to divide
by two than to calculate an exact square root).

--Gary

Gary Drescher wrote:
"Dave S" wrote in message
. net...

Ok... all you closet aeronautical engineers... I'm asking for someone to
help do my work for me.. with regards to Va..

I have an Excel Spreadsheet application that does W&B and plots it on a
graph... The form also lists certain speeds that are "static": Vx/Vy,
Vne, etc.. I would like to modify this form to list Va dependent on
the given calculated gross weight, and perhaps even doctor it up to do
density altitude computations..


Va is proportionate to the square root of the plane's gross weight. Vx,
Vy,
Vl/d, Vs, and Vs1 are also proportionate to the square root of the gross
weight.

A handy approximation is that for small percentages below maximum gross
weight (say, up to 30% or so), the weight-dependent speeds diminish by
half
the percentage that the weight diminishes. So, for example, if you're
20%
below max gross weight, reduce the appropriate speeds by 10%. (The
calculations should be made with regard to CAS rather than IAS, but the
difference is usually small.)

You can find a good explanation of these speeds' weight-dependency at
http://www.av8n.com/how/.

--Gary